Ozone is deleterious to materials and to humans. OSHA's limits for average ozone concentration are up to 0.1 ppm over 8 hours or up to 0.3 ppm over 15 minutes.
Presently, there are a number techniques for measuring atmospheric ozone concentration. One technique employs ultraviolet light absorption whereas another employs the differential creep rubber technique.
The technique of ultraviolet light absorption takes advantage of a 254 nm absorption line of ozone in the electromagnetic spectra and thus measures the concentration of ozone directly. Here, a measured sample of air is pumped into a chamber and illuminated at one end with a low pressure cold cathode mercury vapor ultraviolet light. The ultraviolet light from the mercury lamp has emission at 254 nm. At the opposite end of the chamber is a cesium telluride vacuum diode detector. The determination of ozone is carried out in two steps. Initially, an ozone-free reference gas sample is pumped into the chamber and the transmitted light intensity is measured. Any ozone present in the reference gas is rapidly destroyed by passing the gas over manganese dioxide. In the second step, an "ozone gas sample" is pumped into the same chamber and the transmitted light is measured. The ozone concentration in the "ozone gas sample" can be easily determined by application of a formula.
Ozone detectors which operate on the basis of ultraviolet light absorption can detect as little as about 0.001 ppm of ozone but have the disadvantage of being somewhat large at about 19".times.12".times.6.5"; of being heavy at about 22 pounds; of requiring a full line voltage of 115V; of requiring a warmup time of about 2 hours; of being expensive at about $4,500-$12,000 per detector; and of requiring to be stationary. In short, such detectors are sensitive, expensive and are intended for stationary laboratory use.
The fact that a detector must remain in the lab is a serious disadvantage because ozone concentration often needs to be measured in widely separated locations, such as when one is determining the average ozone concentration over an entire city or when one needs to measure the ambient ozone in every room in a building. Furthermore, another critical disadvantage of an absorption ozone detector resides in the fact that ozone is very chemically active and thus easily destroyed inside many containers, which precludes sample collection.
In a differential creep of rubber technique, a standard rubber thread is divided in half, one part is exposed to the atmospheric ozone whereas the other part is protected from ozone. The unexposed portion creeps at a lower rate than the exposed portion and pulls an indicating needle attached to the exposed rubber along a scale, thus giving a measure of ozone concentration.
For purposes of a reference, summer day ozone concentration in the ambient air at about the ground level is about 0.08 ppm whereas at night it is about 0.02 ppm. In winter, the day ozone concentration is about 0.03 ppm whereas at night it is about 0.02 ppm. In some locations, such as in Los Angeles, ozone concentration exceeds these values. For instance, summer ozone daytime concentration in Los Angeles often exceeds 0.10 ppm whereas at night it is about 0.02 ppm. Since ozone concentration is lower in the morning than in the evening due to the absence of sunlight at night, it is recommended to carry out outdoor activities, such as running, in the morning rather than in the evening to avoid the higher deleterious ozone concentration.